Electrical appliances, such as cellular phones, notebook computers, electronic notebooks, and digital cameras, are desired to have a water-proof function because they often are used outdoors. Parts that are most difficult to be provided with the water-proof function are sound emitting parts and sound receiving parts, such as a speaker, a microphone, and a buzzer. Generally, a housing of the electrical appliance with the sound function has openings at positions corresponding to the sound emitting part and the sound receiving part.
A water-proof sound-transmitting membrane is known as a member for preventing water and dust from entering the housing through the openings provided for the sound emitting part and the sound receiving part while ensuring a satisfactory sound transmittance. The water-proof sound-transmitting membrane is a thin film made of a material that hardly hinders the transmittance of sound. By closing the openings of the housing with the water-proof sound-transmitting membrane, it is possible to attain both the sound transmittance and the waterproofness. As a material suitable for the water-proof sound-transmitting membrane, a polytetrafluoroethylene (PTFE) porous membrane can be mentioned as described in JP 2004-83811 A.
It is known that the waterproofness of the PTFE porous membrane is enhanced by reducing an average pore size. However, a reduced average pore size increases a surface density and lowers the sound transmittance. In short, the sound transmittance and the waterproofness are in a trade-off relationship. Therefore, it is not easy to enhance the waterproofness without lowering the sound transmittance. Then, in Patent Document 1 to be described below, the average pore size and the surface density are specified in order to attain both the waterproofness and the sound transmittance.
In recent years, the level of the waterproofness required for electrical appliances has become higher every year. Specifically, electrical appliances are required to have a waterproofness of not only a level that allows them to be used in daily lives but also a level that allows them to be immersed in water, and furthermore, a level that allows them to be used at a predetermined depth in water for a certain period of time.
It is not assumed that an electrical appliance using the water-proof sound-transmitting membrane disclosed in JP 2004-83811 A is immersed in water. As a result of a series of intensive studies, the following problems have been found in obtaining a water immersible electrical appliance. When a high water pressure is applied to the water-proof sound-transmitting membrane for a certain period of time or longer, the water pressure stretches the membrane. As a result, micropores of the membrane are deformed, allowing water to permeate through the membrane easily, and the membrane tends to rupture easily. Thus, the strength of the membrane is an important factor in attaining a high waterproofness (water resistance) that allows the membrane to withstand even a certain water pressure. When the surface density of the membrane is increased, the strength of the membrane also is increased, but this is accompanied by a decrease in the sound transmittance as described above.